Recently, chemical arrays, more particularly, biomolecular arrays, have been successfully created. For example, Fodor, et al., "Light-directed, Spatially Addressable Parallel Chemical Synthesis," Science, Vol. 251, 767-773 (1991) disclose high density arrays formed by light-directed synthesis. The array was used for antibody recognition. Biomolecular arrays are also described by E. Southern (PCT Publication WO 89/10977) for analyzing polynucleotide sequences. Such biomolecular arrays lend themselves to a large number of applications, from DNA and protein sequencing to DNA fingerprinting and disease diagnosis.
A typical approach for synthesizing a polymer array on an optical substrate is described by Fodor et al. (1991) supra; PCT publications WO 91/07087, WO 92/10587, and WO 92/10588; and US Pat. No. 5,143,854. In such arrays, different receptors are synthesized onto a substrate using photolithographic techniques. Ligands are washed over the array. Either the ligand is fluorescently labeled or an additional fluorescently labeled receptor is also washed over the array. The result is that fluorophores are immobilized on those pixels where binding has occurred between the ligand and the receptor(s). In general, a chemical array is illuminated with radiation that excites the fluorophores. The pattern of bright and dark pixels is recorded. Information about the ligand is obtained by comparing this bright-dark pattern with known patterns of surface bound receptors.
In many application, e.g., in analyzing the human genome, it is often necessary to scan a large number of array elements. Therefore, the ability to read a chemical array with a large number of elements within a short time is highly desirable. Lasers have been used to impinge on chemical array elements with a small spot size beam of high intensity.